An ultraviolet contained in the sunlight is divided into UV-A radiation at 400 to 320 nm, UV-B radiation at 320 to 290 nm, and UV-C radiation at 290 to 100 nm by the wavelength. The UV-A radiation occupies little over than 97% of the total solar ultraviolet amount reaching on the ground, and transmits through a glass or a cloud and permeates to the dermal segment in the back of the skin to cause photoaging such as wrinkle, and slackening.
Conventionally, a responding to UV-B radiation having a strong effect on sunburn has been valued as UV protection. However, further research of photoaging has been done in recent years, and the responding to UV-A radiation captures consumer attention.
For shielding UV-A radiation efficiently, it is needed to combine a large amount of organic ultraviolet absorption agents and/or inorganic ultraviolet shielding agents in products. On the other hand, the organic ultraviolet absorption agent is recognized as a sufficient safety material, but some specific ultraviolet absorption agents are used in limited amounts for cosmetic purpose. From the above, it is needed to shield sufficiently UV-A radiation by using only the inorganic ultraviolet shielding agents.
The inorganic ultraviolet shielding agents such as zinc oxide and titanium oxide to be used in a sunscreen product can reveal the ultraviolet protection performance by the scattering effect of ultraviolet on the powder surface and the effect of absorbing the ultraviolet into the powder particle. The scattering effect depends on the reflection factor of the particle and the particle size, and the absorption effect depends on the band gap energy (Eg) of the powder particle. The Eg of zinc oxide is 3.2 eV and electronic excitation thereof is direct transition so that zinc oxide can absorb effectively the light at the wavelength of 388 nm or less corresponding substantially to the Eg value. On the other hand, rutile type titanium oxide to be widely used in cosmetic use has the Eg of 3.0 eV, but the electronic excitation of titanium oxide is indirect transition so that the light at the wavelength of about 320 nm or less being smaller than 413 nm corresponding to original Eg value can be absorbed effectively.
Further, the Eg of iron oxide (hematite) is 2.2 eV, and the wavelength corresponding to the Eg is 564 nm. Therefore, it is thought that iron oxide absorbs the light at the wavelength of 400 to 564 nm being a visible ray not only the UV-A radiation.
The inventors of the present disclosure completed the invention of hexagonal plate shaped-zinc oxide particle and a method for producing the same (Patent Document 1). The hexagonal plate shaped-zinc oxide particle disclosed in Patent Document 1 has a good smoothness, a soft focus effect, an ultraviolet shielding property, and a visible light transparency, and may be used suitably as an ultraviolet shielding agent for a cosmetic. The zinc oxide particles have more excellent ultraviolet shielding performance than conventional zinc oxide particles, and can be used as an ultraviolet shielding component of a cosmetic. However, it is more preferred to obtain more excellent ultraviolet shielding performance.
An ultraviolet protecting agent composed of titanic acid dizinc fine particles and ultraviolet shielding agent composed of zinc oxide particles in which iron is contained are disclosed in Patent Documents 2 to 5. However, these particles do not have the hexagonal plate shape so that the particles are not particles having the various kinds of performances derived from the above-mentioned hexagonal plate shape.